Liquid-powered centrifugal separators are well known for separating fluids of different densities or for separating particulate matter from liquids and have long been used in lubrication systems for engines and analogous items of vehicles. The common principle of operation is that a housing contains a rotor which is supported therein to spin at high speed about a substantially vertical axis. The rotor comprises a container to which liquid is supplied at elevated pressure along the axis of rotation and is ejected from tangentially directed reaction jet nozzles into the housing from which it drains to the engine sump. Contaminated liquid is also supplied to the rotor to pass therethrough, and in doing so, denser contaminant materials are separated therefrom centrifugally and retained in the rotor. The drive liquid may comprise the contaminated liquid in a so-called self-powered centrifugal separator of the type described, for example, GB-A-735658, GB-A-757538, GB-A-2160796, or GB-A-2383194, or it may be separate liquid as described in GB-A-2297499.
Notwithstanding the origin of the rotor drive liquid, the energy lost by the ejected liquid effects rotation of the rotor about the axis at a speed fast enough for the contaminated liquid circulating in, and passing through, the rotor to deposit solid contaminants on surfaces that are spaced radially outwardly of the axis and face radially inwardly towards the axis. For efficient separation, and to ensure that separated contaminants do not interfere with the reaction jet nozzles, the rotor container may be provided with a radially inwardly extending partition wall that effectively divides the rotor into a separation chamber, in which the solids collect, and an outflow chamber, to which the cleaned liquid passes by way of a transfer aperture cited near the rotation axis. It is common in modem designs, such as, EP 0193000 and GB 2283694, for this partition wall to extend both radially and axially as what is sometimes referred to as a separation cone, which better holds solids within the separation chamber if the rotation axis is tilted from the vertical.
The above identified centrifugal separators are employed principally within pumped lubrication systems of internal combustion engines wherein the contaminants comprise inter alia carbonaceous products of combustion, such as soot particles and the like which tend to bind with the liquid and other solid materials that are retained in the rotor into a dense, cohesive coke-like mass which adheres to upstanding walls of the rotor, even after rotation has ceased, permitting a simple internal structure for the rotor. Even if such material does fall from the rotor wall onto the partition wall, forming it with a slight inclination as a separation cone is sufficient to inhibit the material from being entrained in newly introduced liquid through the annular transfer aperture to the reaction jet nozzles before subsequent rotation applies sufficient centrifugal force to return it to the peripheral wall. However, in the case of liquids which are not contaminated by combustion products the centrifugal separation tends to result in non-cohesive contaminant particles which settle out from the liquid as a wet silt that is relatively mobile and easily washed or entrained by liquid flow as a slurry. It is found that when such non-cohesive contaminants settle out onto the peripheral side wall of a rapidly spinning rotor as a result of centrifugal separation, the non-cohesive nature of the material spreads them out across the vertical wall as a silt of substantially uniform thickness, and that when the rotation speed falls or ceases altogether as a result of ceasing to supply liquid to the rotor the non-cohesive silt slumps downwards and collects on the partition wall. In such a situation, and, notwithstanding any inclination of the partition wall as a separation cone, the non-cohesive silt is readily entrained by subsequently supplied liquid into the transfer aperture before the rotor is caused to spin at such a rate as separate the contaminants from the liquid and hold them within the rotor container against the peripheral side wall thereof.